1. Field of the Invention
The present invention relates to a vane pump suitable for use as hydraulic pressure supply to a power steering device and in particularly, to an improvement in a vane guide arrangement for smoothening radial movements of vanes around at each of ejection ports.
2. Discussion of the Related Art
Heretofore, as a vane pump used as hydraulic oil supply to a power steering device, there has been known one described in U.S. Pat. No. 6,203,303 B1 to H. Fujiwara et al. In this known vane pump, a rotor with plural slits formed to extend radially therein is rotatably provided in a cam ring contained in a pump housing, and a plurality of vanes are slidably received respectively in the slits. A pair of side wall members are provided to close the axial opposite end portions of the cam ring. Plural arc backpressure grooves communicating with innermost end portions of the slits and plural arc communication grooves connecting the backpressure grooves one after another are engraved at an inside end surface of each side wall member on which the rotor rotationally slides, and are supplied with the pump ejection pressure. Further, a recess (or cutout) portion which does not contact the side surface of each vane is formed on at least one of rotor sliding surfaces of the side wall members.
FIG. 4 shows one side plate 1 incorporated in the aforementioned prior art vane pump. The side plate 1 corresponds to a side plate 20 used a vane pump in the embodiment whose longitudinal sectional view is shown in FIG. 1, as referred to later in detail. More specifically, the side plate 1 in the prior art vane pump has a rotor sliding inside end surface, on which there are engraved a pair of right and left suction ports 21 and a pair of upper and lower ejection ports 22. At the center of the side plate 1, an annular backpressure groove 2 is engraved to encircle a through bore 1a which a pump shaft for driving a rotor passes through. The backpressure groove 2 is constituted by a pair of right and left suction backpressure grooves 3 each being radially wide and taking an arc shape, a pair of upper and lower ejection backpressure grooves 4 each being radially wide and taking an arc shape, plural arc communication grooves 5 each being narrow in radial width, and a pair of cutout or recess portions 6. Each of the suction backpressure grooves 3 is arranged radially inside of a corresponding one of the suction ports 21 at an angular or circumferential position close to the same, while each of the ejection backpressure grooves 4 is arranged radially inside of a corresponding one of the ejection ports 22 at a circumferential position close to the same. Each of the communication grooves 5 makes adjoining suction and ejection backpressure grooves 3, 4 communicate with each other. Each of the recess portions 6 is provided to prevent the radially inner part of the side surface of each vane from contacting the inside end surface of the side plate 1 over an angular area where each vane rotating counterclockwise in FIG. 4 remains protruded to the radial outermost position and over another angular area where it is moved radially inwardly (i.e., the pre-compression area and compression area shown in FIG. 4 of the aforementioned United States patent). Further, a passage (not shown) is provided to make each suction backpressure groove 3 communicate with the ejection ports 22. Each recess portion 6 in the prior art technology is formed to cover an angular area which ranges from an angular position slightly leaving the rotationally preceding end of each suction backpressure groove 3 to the rotationally preceding end of each ejection backpressure groove 4. The axial depth of each recess portion 6 is the same degree as those of the communication grooves 5 and is shallower than those of the suction and ejection backpressure grooves 3, 4. The outer circumferential edge of each recess portion 6 is an arc shape which is somewhat larger in radius than the locus (L) drawn by the radially innermost end of each vane and takes its center on the center of the through bore 1a. 
In the vane pump of the aforementioned prior art technology, the contact length of the side surface of each vane with the inside end surface of the side plate 1 is shortened by providing the recess portion 6. As a result, sliding resistance is decreased between the inside end surface of the side plate 1 and the side surface of each vane as well as between each vane and the rotor slit receiving the same. Further, an increased pressure in each ejection backpressure groove 4 which is attributed to the radially inward movement of each van is applied by way of the recess portion 6 to the radial innermost end of the vane in an angular area where the same is to be protruded to the outermost position, so that the vane can be enabled to protrude quickly from the rotor at the start of the pump operation.
In vane pumps of this type, the radial movement of each vane supported in the rotor is done with the side surface thereof being slidably guided on the inside end surface of the side plate 1 in the same manner as shown in FIGS. 1 and 2 of the accompanying drawings. In an angular area over which each ejection port 22 elongates, however, the inside end surface of the side plate 1 for slidably guiding and supporting the side surface of each vane which is radially inwardly moved by being pressed along the cam surface of the cam ring upon rotation of the rotor defines a guide area, whose radial width is almost constant and narrow, between the outer circumferential edge of each arc shape recess portion 6 and the inner circumferential edge of each arc shape ejection port 22, as indicated at “e” in FIG. 4.
As discussed above, in the prior art technology, the inside end surface of the side plate (side wall member) 1 which slidably guides the side surface of each vane in the angular area over which each ejection port elongates is the guide area (e) whose radial width is almost constant and narrow. This tends to cause each vane passing there to incline relative to the side plate 1, and thus, the side plate 1 is insufficient to guide each vane. On the other hand, the force which causes each vane rotating with the rotor to move radially inwardly within the same angular area is generated by sliding the radial outer end of each vane on the slanted cam surface at the internal surface of the cam ring against the friction force therebetween. Therefore, the force tends to involve self-induced vibration and varies irregularly in terms of time and place. In this way, as the irregularly variable force is exerted on each vane guided by the guide surface which is insufficient in the function therefor, each vane is inclined relative to the guide area (e) to scrape against the same and cannot move stably and smoothly. For this reason, there arises a problem that the cam surface at the internal surface of the cam ring is abraded notably within the aforementioned angular area, thereby generating pulsation in the pump ejection pressure as well as increasing the operation noise.
A similar problem arises in the case where the backpressure groove 2 is of the type that the recess portions 6 are omitted to constitute the backpressure groove 2 only by the suction and ejection backpressure grooves 3, 4 and the communication grooves 5. In this modified case, within the angular area over which each ejection port 22 elongates, the inside end surface of the side plate 1 which slidably guides one side surface of each vane when the same is moved radially inwardly defines a guide area whose radial width is almost constant and narrow (though somewhat wider than that on the aforementioned guide area (e)) between the outer circumferential arc edge of each ejection backpressure groove 4 and the inner circumferential arc edge of each ejection port 22. Accordingly, the similar problem results in smaller damage than that in the aforementioned case, but remains left unsolved.
In addition, the similar problem arises in the case of a further modified side plate wherein the backpressure groove 2 is formed by extending the suction backpressure grooves 3 (or the ejection backpressure grooves 4) continuously over the whole circumferential length.